Various operating mechanisms are known that provide a cord drive for raising a covering, such as a Venetian blind or pleated shade, relative to an architectural structure (e.g., a window). Examples of such cord drives are disclosed in U.S. Patent Publication No. 2009/0120592, filed on Nov. 3, 2008, and U.S. Patent Publication No. 2009/0120593, filed on Jan. 16, 2009, the disclosures of both of which are hereby incorporated by reference herein in their entirety for all purposes. In embodiments of the operating mechanisms disclosed in such publications, the cord drive is housed in a plastic housing, and short strokes of a drive cord unwinding from a cord drum raise the covering. Additionally, a spring is used to rewind the cord onto the cord drum after each stroke and a Bendix-type mechanism is used to drive an output shaft in connection with each short cord stroke, with rotation of the output shaft raising the covering. When the cord is released, the drive is disengaged to allow the spring to rewind the drive cord onto the cord drum. Thereafter, when the user again pulls on the drive cord, the Bendix-type mechanism re-engages to rotate the output shaft in a manner that further raises the covering. This process is then repeated until the covering is raised to the desired position.
The Bendix-type mechanism typically includes axially-projecting gear teeth on an input member that is configured to move axially with each drive cord stroke to engage mating axially-projecting gear teeth on an output member of the mechanism in order to drive the output shaft. In addition, the Bendix-type mechanism typically includes a drive member that rotates with the cord spool. The drive member is configured to rotate relative to the input member until an axial projection on the drive member engages an internal face on the input member to rotate the input member and push such member axially to cause the input member to engage with and rotate the output member.
Such a cord drive arrangement requires that there be some amount friction on the input member to prevent rotation of the input member relative to the drive member until the axial projection of the drive member “catches up” to or otherwise engages the internal face on the input member to push it forward and provide the typical Bendix-type mechanism action. In prior art devices, this friction has been obtained by using a plastic arm on the housing that encases the cord drive. The plastic arm contacts the input member and acts as a spring, applying a small spring force to the input member to create friction between the plastic arm and the input member to prevent the input member from rotating with the drive member when it is not being positively driven by the drive member. Unfortunately, given that the plastic arm is held in a stressed condition for a very long period of time, the plastic material may cold flow over time, thereby reducing the spring force provided by the plastic arm such that the plastic arm no longer prevents the input member from rotating with the drive member when it is not being positively driven by the drive gear.
Accordingly, an improved cord drive assembly that allow for a more constant, reliable restraining or braking force to be applied to the input member of an associated Bendix-type mechanism of the cord drive assembly would be welcomed in the technology.